FIGS. 1A and 1B illustrate the configuration of an optical module described in Japanese Patent Application Laid Open No. 2013-140211 as a related art example of this type of optical module. The optical module includes a circuit substrate 12 on which one or more optical elements 11 are mounted, a housing 14 which accommodates the circuit substrate 12 therein, an optical fiber holding member 16 which holds one or more optical fibers 15 introduced from an optical cable 13 into the housing 14, and an optical coupling member 17 which optically connects the one or more optical elements 11 on the circuit substrate 12 and the one or more optical fibers 15. An end part of the optical cable 13 is fixed on the housing 14. The optical coupling member 17 is fixed on the circuit substrate 12. The optical coupling member 17 and the optical fiber holding member 16 are mutually coupled by fitting a guide pin 17a of the optical coupling member 17 into a guide hole 16a of the optical fiber holding member 16. The number of the optical fibers 15 is generally equal to the number of the optical elements 13, and the number of the optical elements 13 is usually greater than or equal to 2.
The optical coupling member 17 covers the optical element 11. Lens surfaces 17b are formed on a surface, which is opposed to the optical fiber holding member 16, of the optical coupling member 17. The number of the lens surfaces 17b is equal to the number of the optical elements 13. Lens surfaces (not shown) are formed on a surface, which is opposed to the optical element 11, of the optical coupling member 17. The number of these lens surfaces is equal to the number of the optical elements 13. A reflection surface 17c is formed on an upper surface of the optical coupling member 17.
In the case where the optical elements 11 are light emitting elements, for example, rays of light emitted from the light emitting elements pass through the lens surfaces to be incident on the optical coupling member 17. The rays of light reflected by the reflection surface 17c are optically coupled with end surfaces of the optical fibers 15 by the lens surfaces 17b. 
Thus, the optical module illustrated in FIGS. 1A and 1B optically connects the optical elements 11 with the optical fibers 15. In this example, both of an optical connection direction and a mechanical connection direction between the optical fiber holding member 16 and the optical coupling member 17 are parallel to a plate surface of the circuit substrate 12.
As described above, since both of the optical connection direction and the mechanical connection direction between the optical fiber holding member 16 and the optical coupling member 17 are parallel to the plate surface of the circuit substrate 12 in the optical module of the related art illustrated in FIGS. 1A and 1B, a low-profile optical module can be configured. However, it is not easy to assemble the optical module. It is necessary to slide the optical fiber holding member 16 in an extending direction of the optical fibers 15 (that is, in a direction parallel to the plate surface of the circuit substrate 12) in assembling of the optical module illustrated in FIGS. 1A and 1B. Therefore, in the case where the optical fibers 15 are fixed at the end part of the optical cable 13, for example, the optical fibers 15 need to be bent and this bending may cause a damage of the optical fibers 15.
Further, the optical fiber holding member 16 and the optical coupling member 17 are mechanically connected to each other by inserting the guide pin 17a into the guide hole 16a. An insertion direction of the guide pin 17a is parallel to the plate surface of the circuit substrate 12. The optical fiber holding member 16 is positioned at one end side (that is, bonding end surface side) in the Z direction (that is, a direction parallel to the plate surface of the circuit substrate 12) by two guide pins 17a aligned in the X direction. Definitions of three orthogonal axes of X, Y, and Z are illustrated in FIGS. 1A and 1B. In the case where a force in the Y direction (a direction orthogonal to the plate surface of the circuit substrate 12), for example, is applied to the optical fibers 15 (or the optical cable 13), the optical fiber holding member 16 may rotate about the X axis and accordingly, optical coupling performance may be degraded.
By elongating each of the guide pin 17a and the guide hole 16a, for example, rotation about the X axis of the optical fiber holding member 16 can be suppressed. However, when the guide pin 17a is elongated, assembling becomes further difficult.